Background: I have a mainform that is linked to a table...also on this form is a combobox that allows the user to scroll through all available records on pieces of equipment and select the one he wants to come up.
On this form is a subform dealing with service dates on this piece of equipment. The mainform has unit numbers as the primary key and the subform uses the unit number and date of service as primary keys.

I have two main problems:
1. When I try to enter (as a user) the service date in the subform I get an error message stating "You can't assign a value on this object (object may be control on read only form, object may be on form open in design view, value may be too large for the field)." Once I click ok on the message I am able to continue entering the date.

2. Even when all the fields contain appropriate data, I get an error message stating that the "Index or primary key cannot contain null value."

I've made a very similar set of forms before, and have never had problems like this...any suggestions as to what I could be missing? I'm new to Access (2K) and don't know much VBA yet.